1. Field of the Invention
The present invention relates to a light-emitting device, and more particularly to a surface emitting type light-emitting device.
2. Description of the Prior Art
Small, light, light-emitting devices are extensively used in various fields. In recent years, light-emitting devices are also used in optical printers that use a beam of light to record information, in image and bar-code reading systems that utilize the intensity of a reflected beam of light, and in optical communications devices that utilize optical signals.
FIG. 6 is a cross-sectional view of the structure of a conventional light-emitting device, and FIG. 7 is a plan view of a conventional light-emitting device. With reference to the drawings, GaAsP containing tellurium is used to form an n-type semiconductor 2. Zinc is diffused into the n-type semiconductor 2 to form p-GaAsP semiconductor (hereinafter referred to as p-type semiconductor 1). A positive electrode (p-electrode) 3 is then provided on the upper surface of the p-type semiconductor 1 and a negative electrode (n-electrode) 4 on the lower surface of the n-type semiconductor 2. When a forwardly biased voltage is applied to the junction between the p-type semiconductor 1 and the n-type semiconductor 2, the electrical energy is converted into optical energy by the recombination of the carriers injected into the junction, producing an emission of light.
However, when a forwardly biased voltage is applied to a p-n junction, the relationship between the current flowing in the light-emitting device and the optical output is as shown by the solid line in the graph of FIG. 8. Namely, while the optical output increases when the current is increased, in low current regions this increase is nonlinear and depends strongly on the current. That is, when a voltage V is applied to the p-n junction, the device current is divided into a component that is proportional to exp(eV/kT) (hereinafter referred to as the type A current component) and a component that is proportional to exp(eV/2kT) (hereinafter referred to as the type B current component). In this case, the intensity of light emitted by the light-emitting device is proportional to the type A current component current.
Since in a high current region the type A current component accounts for the major portion of the total current, the amount of light emitted by the light-emitting device is proportional to the type A current component and the increase in the light amount is linear with respect to the increase in the device current.
That is, when there is a large current there is a linear relationship between optical output and current that does not depend on the current, but when the current is small the type B current component occupies a relatively large proportion and the relationship between the A and type B current components depends on the voltage, so the increase in light intensity relative to the increase in current becomes nonlinear. The type B current component current consists mainly of electron and hole recombinations which occur in the depletion layer in the vicinity of the p-n junction exposed on the device surface.
In a light-emitting device, an increase in light intensity that is nonlinear with respect to the increase in current presents problems when it comes to using the current to control the light intensity, making it difficult to control the light intensity when such light-emitting devices are used as light sources of optical printers, image readers and optical communications devices.